The northern Cephissus river (Greek Κήφισσος: Kifissós, Kephissós, or Kêphissos) or Cephisus (Greek Κήφισος: Kêphisos) rises at Lilaea in Phocis and flows by Delphi through Boeotia and eventually issues into Lake Copais which is therefore also called the Cephisian Lake. This Cephissus is the modern Mauro Potamo.

Pausanias (9.38.7) records a Theban tradition that the river Cephissus formerly flowed under the a mountain and entered the sea until Heracles blocked the passage and diverted the water into the Orchomenian plain; but he does not believe it.

Pausanias (10.8.1) also says that the Lilaeans on certain days threw cakes and other customary items into the spring of the Cephissus and that they would reappear in the spring of Castalia.

The same author names as daughters of this Cephissus:

• the naiad Lilaea (10.33.4) the emponym of Lilaea at its source,
• Daulis (10.4.7) the eponym of the city of Daulis,
• Melaeno (10.6.4) mother of Delphus by Apollo, though he also gives two other accounts of Delphus' mother. However one of these alternate versions is that Thyia daughter of the aboriginal Castalius was Delphus' mother, almost certainly the same Thyia whom Herodotus (7.178.1) claims was daughter of Cephissus to whom the Delphians built an altar to the winds and who was eponym of the Thyiads .

A mortal son of Cephissus was Eteocles by Euippe daughter of Leucon son of Athamas. This Euippe was wife of King Andreus of Orchomenus and Eteocles inherited Andreus' throne (9.34.9). Eteocles or Eteoclus son of Cephissus is confirmed from Hesiod's Catalogue (Fr. 26) and Pindar (Ol. 14). He first made offering to the Charites by the side of the river Cephissus.

Cephissus was also father of Narcissus according to Ovid's Metamorphoses (3.342), Hyginus (271), and Statius' Thebaid (7.340), Narcissus' mother being an otherwise unknown naiad named Liriope according to Ovid.